Aluminum storm window, plastic corner

Storm window has 4 of these plastic corners. They seem to be held in by a tiny screw and locks into the corner. It seems to have a metal weld through the corner so I'm having trouble removing it completely. Someone told me there's probably a way of hammering the metal out of the frame but I don't want to try that in case I break it. The glass had broken and when someone tried to take the frame apart to replace it, he wasn't familiar with these corner pieces and three of them broke and he said these are no longer available anywhere. The storm windows don't seem to have any markings on them. We bought the house in 1984 and these were in the windows. The house was built in 1958-1959 but I don't know if they came with the house or were later put in. Tried to get as good a picture as I could. Length is about 1-1/2 inches and width looks about 1/4 inch.